1. Field of the Invention
This invention relates to a recycle chromatograph with supercritical fluid which separates a mixture into components by recycling a mobile phase through a separation column repeatedly until the mixture is separated sufficiently, whereby a desired component can be fractionated.
2. Description of the Prior Art
Carbon dioxide is the substance most widely used as the mobile phase in supercritical fluid chromatography. Although carbon dioxide exhibits solubility when it is a liquefied gas or a supercritical fluid, i.e., its density is liquid like density 0.3 to 1.2 g/ml, the solubility thereof with respect to the solute becomes substantially zero when the pressure is reduced to atmospheric pressure, whereupon it becomes a normal gas. Therefore, the solute of a sample can be easily separated from the fluid with high purity when carbon dioxide is used as the mobile phase in supercritical fluid chromatography.
However, many conventional supercritical fluid chromatographs are designed to use only as an analyzing device, but not as a fractionating device, that is, a device for separating and purifying the desired component contained in a sample mixture. The major reason for this lies in that when fractionation is intended, a proper column separation efficiency needs to be realized by using a relatively long column with large diameter for high sample loading capacity, resulting in high mobile phase flowrates.
In order to overcome the above-described problem, a recycle method is widely used in liquid chromatography. This recycle method is a method in which the column's effective length is increased by repeatedly-passing the mobile phase containing solute through the column via the mobile phase delivery pump, the separation or elution takes place without any solvent consumption, and a portion of column effluent containing a desired component can be drawn out for fractionation in which period the same amount of solvent can be fed. If this recycle method can be applied to a recycle chromatograph with a supercritical fluid as the mobile phase, its applicable range can be widened.
It is well known that, in SFC, a high pressure drop causes density decrease along the column toward outlet, resulting in reducing the efficiency. Therefore, the obtainable plate number is not in proportion to the column length.
In this regard, the recycle operation of SFC is very favorable, because the column efficiency is increased by a factor of the number of recycles without increasing the back-pressure, if extracolumn band broadening is negligible.